Previous studies from our group and others have identified a critical role of the chemokine CXCL12/SDF1 and its receptor CXCR4 in the retention and mobilization of myeloid cells from the bone marrow. CXCL12 is abundant in the non hematopoietic cells of bone marrow, particular in the CAR (CXCL12 abundant reticular) and in other mesenchymal cells in the bone marrow. The bone marrow hematopoietic cells generally express CXCR4, and the signaling interaction between CXCL12/CXCR4 serves to retain hematopoietic cells in the bone marrow. G-CSF is a critical physiological regulator of granulopoiesis: mice carrying homozygous deletions of colony-stimulating factor (G-CSF) or its receptor are severely neutropenic, and dominant-negative mutations of G-CSFR have been linked to severe defects of granulopoiesis. Administration of G-CSF induces an expansion of myeloid lineage cells in the bone marrow, and promotes the release of mature myeloid cells and hematopoietic progenitor cells from the bone marrow to the peripheral blood. Thus, G-CSF at pharmacologic concentrations is widely used to induce granulopoiesis and to mobilize hematopoietic progenitors to the peripheral blood. Functionally, G-CSF reduces CXCR4 and CXCL12 levels in the bone marrow through a variety of mechanisms. As a consequence, the glue that retains hematopoietic cells in the bone marrow is decreased and massive exit of mature and immature hematopoietic cells is achieved. A CXCR4 competitive inhibitor, AMD3100/Plerixafluor, has been approved by FDA and a mobilizing agent for hematopoitic precursors in conjunction with G-CSF. Recently, our studies on HSPC mobilization have detected an important role of the receptor/ligand pair EphrinB2/EphB4. We unveiled the mutually exclusive bone marrow distribution of EphB4 receptors in the sinusoids and EphrinB2 ligands in hematopoietic cells, and discovered an EphB4/EphrinB2-dependent pathway that controls HSPCs mobilization. Blockade of EphB4/EphrinB2 reduced HSPCs and other myeloid cells mobilization to the circulation. In murine cancer models, in which hematopoietic cells derived from the bone marrow promote tumor growth, EphB4/EphrinB2 blockade reduced tumor infiltration with HSPCs and tumor progression. These results identify EphB4/EphrinB2 signaling as critical to hematopoietic cells mobilization from bone marrow and provide a new strategy for reducing cancer progression by targeting the bone marrow. Other ongoing studies are focused on the contribution of EphrinB2 signaling in the HSPC to the regulation of CXCR4 signaling in the same cells. Previous studies in neuronal cerebellar cells have defined a biochemical pathway by which PDZ-dependent signaling by EphrinB2 recruits the PDZ-containing RGS3 protein, which in turn can bind and shut down CXCR4 signaling. Proof of principle experiments have identified association of EphrinB2 and active CXCR4 in the mouse bone marrow cells. We are currently mapping the sites where these molecules associate in the bone marrow and beginning in vitro experiments to study the biochemical output of this interaction.